The present invention relates to an acoustic absorber and to a sound-absorption method for damping sound in a fluid flowing in a duct.
Acoustic absorbers are used to reduce sound levels or absorb sound. For example, acoustic absorbers are used to reduce the sound level in aircraft engines, in exhaust gas systems, in air-conditioning systems and in general in fluid flow ducts.
Known acoustic absorber elements, such as liners in aircraft engines or resonators, are made of passive structural members and therefore cannot be adapted during operation, to varying conditions of the sound field. Thus, the known acoustic absorber elements cannot be adapted during operation to altered conditions of the sound and the fluid flow, such as acoustic pressure, fluid composition, flow velocity, turbulence, and the like. With conventional absorbers, therefore, it is not possible to achieve optimal reductions of sound level under different operating conditions.
Adaption by means of electrodynamic, piezoelectric actuators would have the disadvantage of a very complex closed-loop control system. Furthermore, because of their poor robustness, such actuators have only very limited usability in acoustic absorbers.
International Patent WO 92/15088 describes an acoustic damping device in which a resonator can be adjusted to the frequency of the acoustic vibrations. For this purpose, there is disposed in the resonator a movable partition, which can be driven by a drive rod and motor in order to vary the volume in the resonator.
To vary the resonance frequency, it is proposed in German Patent 4228356 C2 that there be provided a resonator neck of variable length and cross section in the form, for example, of a flexible hose, which is stretched by means of a positioning device or a pulling mechanism.
Unfortunately, the known devices and methods suffer from the problem of great structural complexity in order to make the resonator volume adaptable. Furthermore, such systems often lack the necessary robustness with respect to mechanical stresses and strains.
An object of the present invention is therefore to provide an acoustic resonator which can be adapted to different acoustic conditions, requires relatively little structural complexity and can be designed robustly.
Another object is to provide a sound-absorption method by which an optimal reduction of sound level can be achieved in simple manner under different operation conditions.
These and further objects are achieved by an acoustic absorber comprising a wall defining an interior chamber, the wall having a duct therethrough for conveying a fluid, the wall having an opening providing communication between said duct and said chamber. The chamber has a natural frequency and acoustic impedance to absorb sound in the fluid in the duct. The natural frequency and acoustic impedance of the chamber is adjustable by means of an elastic membrane for varying volume of the chamber and/or a heating element for heating the chamber.
The method is achieved by the steps of connecting a chamber of a sound absorber, to said duct and varying a natural frequency and impedance of said chamber to adjust the sound absorber to the sound in said duct, the varying of the natural frequency and impedance being effected by varying the volume of said chamber by a membrane acting on said chamber, and/or by varying the temperature of the chamber by a heating element disposed in said chamber.
Because of the variable natural frequency and impedance of the absorber, the absorber can be adapted optimally to the surrounding sound field, even during operation under different operating conditions. Thus, an optimal reduction of sound level is achieved. For this purpose a closed-loop control system of only minimal complexity is necessary.
Advantageously the natural frequency of the absorber and the impedance of the absorber can be adjusted mechanically and/or thermally. Thereby optimal adaptation to the surrounding acoustic field can be achieved in particularly simple manner. Furthermore, the acoustic absorber can also be constructed inexpensively. The absorber impedance can be adapted with respect to reactance and resistance.
Preferably, the acoustic absorber comprises an elastic membrane for varying the volume of the absorber chamber. In this regard, the membrane can be acted on pneumatically or hydraulically. Thereby optimal adaptation of the absorber during operation can be accomplished by simple structural means, thus additionally allowing a greater degree of robustness to be achieved.
Advantageously, the acoustic absorber has a heating element for heating the absorber chamber. Such a heating element can be, for example, a zig-zag heating wire or a helical heating wire. This embodiment is particularly robust and inexpensive.
Preferably, the acoustic absorber also comprises a device for varying the flow resistance in the region of the duct openings. Thereby, there can be achieved an even higher degree of adaptation and thus an even further improved reduction of sound level.
As such a device, the acoustic absorber can comprise, for example, a heatable wire screen or wire braid, which is advantageously disposed in the region of the duct openings. Thus, the flow resistance in the region of the openings can be varied by heating the wire screen, thus leading to adaptation of the resistance of the absorber. During heating of the wires or of the wire screen there takes place, for example, a volume variation, which in turn results in a variation of flow resistance. By this feature also there can be achieved even better adaptation of the absorber to the acoustic field.
Advantageously, the acoustic absorber includes a sensor unit for temperature measurement. The sensor unit can be disposed, for example, on the heatable wire braid or wire screen, or on the heating wire in the chamber. Thereby, open-loop control or even closed-loop control of the operating frequency or of the acoustic resistance can be achieved with simple means.
Advantageously, the acoustic absorber is equipped with an open-loop or closed-loop control unit for adjustment of the temperature and/or of the absorber volume.
In a further embodiment, a microphone is additionally provided in the chamber of the absorber in order to detect the working frequency of the absorber and use it as the manipulated variable for a closed-loop control system.
In the sound-absorption method, the volume of the absorber chamber is coupled to the duct in order to absorb a sound field in the duct, the natural frequency and impedance of the absorber being varied in order to adapt the absorber to the sound field in the duct. Thereby, an improved or optimal reduction of sound level can be achieved even under different operating conditions or acoustic properties in the duct. The working frequency range and/or the acoustic impedance of the absorber can be adapted during operation to varying acoustic and flow-related acoustic properties of the sound field to be damped.
Advantageously, the size of the absorber chamber is varied by means of a membrane. In addition, or as an alternative thereto, it is possible to adjust the temperature of the absorber chamber by a heating element under closed-loop control.
Advantageously, the flow resistance between the duct and the absorber chamber can be adjusted, for example, by a heatable wire screen or other means with equivalent effect.
In addition to the advantages described hereinabove, the absorber of the invention can be manufactured simply and inexpensively. The acoustic absorber has light weight and operates reliably even under adverse environmental conditions, such as very high or low temperatures, highly or slightly turbulent flow, greatly fluctuating acoustic pressures and similar constraints. The complexity of the closed-loop control system is slight and the acoustic absorber can be used advantageously for the most diverse technical purposes, such as in aircraft engines, in motor vehicles, in exhaust systems, in air-conditioning systems, and the like.